The invention generally relates to communication systems. More specifically, the invention relates to digital switching systems. Yet more specifically, the invention relates to arrangements for regulating traffic in digital switching systems.
Complex communication systems are usually realized as multi-processor systems. Multiprocessor systems can have advantages as compared to traditional mono-processor systems such as, for example, a marked increase in the processing speed. This increased processing speed is achieved by parallel processing of a plurality of tasks that respectively represent sub-tasks of a higher-ranking task; every processor works on a sub-task and the sub-results thereof are in turn combined to form an overall result for the higher-ranking task.
In order to accommodate the demands connected therewith, the internal system executions of a plurality of processors must be controlled and monitored. The complexity of the tasks and processors sequencing in a communication system is thus markedly increased. A complex interplay of software processes is thus necessary.
To produce order in such a complex interplay of processes, the individual processes usually have allocated to them what are referred to as priority levels. This means that only a process that momentarily has the highest priority level is allowed to sequence. Processes of a lower priority level are prevented from being executed until the processes of higher priority levels have been run.
The software of a communication system generally is composed of modularly structured, task-related software processes. These processes then generally are divided into switching dependability and operating applications processes.
The processes are then assigned to respective processors that assume the handling of the assigned tasks.
The switching applications and the dependability application processes generally have high priority levels. The dependability applications processes should sequence when units of the communication system have failed. In this case, measures for the elimination of occurring errors and for restoring the functionability of the appertaining units must be immediately initiated. For this reason, higher priority levels are allocated to the dependability applications processes than to the switching-oriented processes. Switching-oriented processes that are already sequencing are interrupted by dependability-oriented processes.
For dependability reasons, central processor units in particular are redundantly present in a communications system. Given outage of one central processor unit, thus, an immediate switch can be made to a redundant unit without incurring noteworthy losses in dynamics.
This is not the case with the peripheral equipment of a communication system. Due to the plurality of peripheral assemblies providing interfaces to terminal equipment, redundancy would be uneconomical. Since the peripheral equipment are not redundantly present, these assemblies must be monitored all the more exactly and the central processor units must be immediately informed of a total outage of a peripheral device. The central processor units can then in turn immediately initiate countermeasures, such as transmitting dependability-oriented programs/data from external storage units connected to the central units to the peripheral assemblies. There is the risk, however, that the momentarily sequencing processes of the switching applications will be displaced by the dependability-oriented processes executing the transmission of programs/data to the peripheral assemblies and, thus, the chronological execution of the switching-oriented processes will be delayed--particularly in times of a high switching-oriented workload, i.e. at times wherein a plurality of switching-oriented processes are sequencing and the processors and memory units assigned to them are burdened up to capacity. A displacement of the switching-oriented processes, however, is to be avoided under all circumstances since this can mean in practice that connections to be newly set up between two terminal subscribers cannot be set up in a system working at capacity.
This problem was resolved before by subjecting the dependability-oriented processes to what is referred to as a "timer control". A timer control ensures that a dependability-oriented process can only proceed to execution when a timer allocated to it has expired, this being usually reported by a "timer interrupt"; in this case, the switching-oriented processes are immediately interrupted since the higher prioritization of the dependability-oriented processes takes full effect. Such a procedure has the advantage that the switching-oriented processes sequence remain undisturbed during the "timer running" period and can only be interrupted by dependability-oriented processes upon "timer expiration".
This arrangement, however, is disadvantageous in that the transmission of programs and data of the dependability applications uses too much time since a transmission is only punctually implemented, i.e. respectively upon expiration of a timer; the demand for immediately initiated counter-measures in order to immediately place down units back into operation can thus be only conditionally met. Since, further, the processes implementing a transmission of switching-oriented/dependability-oriented program/data are controlled by the central processor units, this arrangement places an additional load on the central processor units when called to effect the foregoing.